A well established method of manufacturing glass bottles and the like is the "press and blow" method in which a gob of molten glass is supplied to a parison mould, a plunger is pressed into the glass to provide a hollow parison, and the hollow parison is transferred to a blow mould to be blown into a bottle of the required final shape.
The pressing operation is one which is critical to the quality of the eventual bottle, and much consideration has in recent years been given to precise control of the movement of the plunger and also to plungers of various constructions.
In European Patent Application 405387, a plunger is described which is of open pore material, specifically sintered metal, and a parison forming operation described in which while the plunger is pressing molten glass into a parison mould, a predetermined quantity of water is supplied to the interior of the plunger, out through the pores of the plunger, to provide a separating vapour layer between the plunger surface and the glass.
The parison forming operation carried out according to the instructions in EP-A-0405387 shows some significant advantages over a conventional parison forming operation using a solid metal plunger, in particular in that the interior surface of the parison (and, consequently of the blown bottle) is remarkably free from minute defects, the presence of which reduces the strength of the blown bottle. However, the operation also suffers from a major defect, which reduces the commercial usefulness of the operation, and which is that bottles formed by this method are particularly prone to show defects (checks) in the neck portions of the bottles, which defects renders the bottles commercially unacceptable.
Attempts to overcome this problem with porous plunger by use of a plunger of variable porosity, or by use of an impermeable barrier around the plunger in the region of the neck area of the plunger are described in DE 4028005 and DE 4028028, but neither have been sufficiently successful to make the use of a porous plunger to provide a separating vapour layer between the plunger surface and the glass in the formation of a parison a commercially successful process.
It is an object of the present invention to provide an improved plunger assembly for use in forming parisons in a glassware forming machine.
It is another object of the present invention to provide an improved method of forming a parison in a glassware forming machine.